In many integrated optical devices, signals are carried within waveguide channels, which are formed by modifying the surface of a substrate. If the waveguide is optically active, the substrate material is in many occasions anisotropic, usually being a crystal having the ability to rotate the plane of polarization of light passing therethrough. Electro-optically active waveguides have electrodes formed in the close vicinity thereof.
The fundamental phenomenon that accounts for the operation of electro-optic modulators and switches is the change in the index of refraction produced by the application of an external electric field. When an electric field is applied across an optically active medium, the distribution of electric charge within it is distorted so that the polarizability and, hence, the refractive index of the medium changes anisotropically. The result of this electro-optic effect may be to introduce new optic axes into naturally doubly refracting crystals, or to make naturally isotropic crystals doubly refracting. In the most general case, this effect is non-isotropic, and contains both linear (Pockels) and nonlinear (Kerr) effects. In commonly used waveguide materials, the nonlinear (quadratic) Kerr electro-optic coefficient is relatively weak.
Thus, an electro-optic crystal will in general exhibit birefringence, if an electric field is applied in a given direction. The most general expression for the linear change in the refraction index ellipsoid components due to the application of an electric field E is given by:             Δ      ⁡              (                  1                      n            2                          )              i    =            ∑              j        =        1            3        ⁢                  r        ij            ⁢              E        j            where i=1,2,3,4,5,6 and where j=1,2,3 are associated with X,Y,Z respectively, the 6×3 matrix [rij] being the electro-optic tensor.
Owing to the fact that substrate materials with a strong electro-optic effect are inherently non-isotropic, the functional parameters of the devices will depend on the polarization state of the light propagating within the medium. Such polarization dependence of functional parameters is one of the main limitations of many integrated optical devices based on substrates of low crystal symmetry. As well known, practically all the presently installed fiber-optic infrastructures consist of standard single-mode fibers that do not preserve the state of polarization of the transmitted light. LiNbO3 material has a mature technology for the processing of integrated optical devices that is nowadays implemented routinely in commercial products, most of them being, however, polarization dependent. This fact limits the scope of application of this technology to cases where the device is placed directly following a polarized laser source, or alternatively, implies utilization of costly polarization-maintaining fibers in the network.
The present trend of all-optical networking dictates the need for polarization-independent devices. It is therefore of prime interest to develop configurations that allow polarization independent functioning of devices.
Attempts that have been made to provide polarization-independent modulators generally utilize two different approaches. The first approach is based on independent electro-optic control of the modulation of both polarizations. According to this technique, specific elements of the electro-optic tensor are used for separately modulating TE and TM modes propagating along the waveguide. Devices of this kind typically require two independent electrode sets to provide the desired electric field for both TE and TM polarizations. This approach is disclosed, for example, in the following publications:
(1) J. Ctyrocy et al., “Two-mode-interference Ti:LiNbO3 electrooptic polarization independent switch or polarization splitter”, Electron. Lett., Vol. 23, No. 27, pp. 965–966, 1991; and
(2) N. Kuzuta, K. Takakura, “Polarization insensitive optical devices with power splitting and switching functions”, Electron. Lett., vol. 27, No. 2, pp. 157–158, 1991.
According to the second approach, the functional dependence on the state of polarization of the input light is eluded by using a specific orientation of the crystal and propagation direction of light signal. This is disclosed, for example, in the following publications:
(3) J. Saulnier et al., “Interferometric-type polarization splitter on Z-propagating LiNbO3:Ti”, Electron. Lett., vol. 26, No. 23, pp. 1940–1941, 1991.
(4) Ed. J. Murphy et al., “Low voltage, polarization-independent LiNbO3 modulators”, Proc. 7th Eur. Conf. on Int. Opt. (ECIO'95), pp. 495–498, 1995);
(5) J. Hauden et al., “Quazi-polarization-independent Mach-Zehnder coherence modulator/demodulator integrated in Z-propagating Lithium Niobate”, IEEE Journal of Quantum Electronics, vol. 30, No. 10, pp. 2325–2331, 1994.
According to the disclosures in the above documents (3), (4) and (5), the direction of propagation of the waveguide was parallel to the optical axis Z, and the largest electrooptic coefficient r33 of LiNbO3 was not used. Here, polarization-independent action is obtained at the cost of larger operating voltages or the device's length. Moreover, in these configurations, voltage induced polarization rotation is unavoidable in LiNbO3 due to the appearance of the r51 coefficient. This effect causes difficulty in the insertion of such a phase modulator in a Mach-Zehnder scheme.
According to another technique, disclosed in C. C. Chen et al. “Phase correction by laser ablation of a polarization independent LiNbO3 Mach-Zehnder modulator”, IEEE Photonics Technology letters, vol. 9, No. 10, pp. 1361–1363, October 1997, a residual phase correction between TE and TM electrooptic responses is obtained by the laser ablation process, decreasing the effective indices of the modes in one of the interferometric arms.
Yet other developed techniques are based on coherence modulation (H. Porte et al., “Integrated waveguide modulator using a LiNbO3 TE−TM converter for electrooptic coherence modulation of light”, Journal of Lightwave technology, vol. 6, No. 6, pp. 892–897, 1988), or on the use of a multi-electrode configuration utilizing the r51 component of the electro-optic tensor (W. K. Burns et al., “Interferometric waveguide modulator with polarization independent operation”, Appl. Phys. Lett, vol. 33, No. 11, pp. 944–947, 1978). Generally speaking, most of the known techniques aimed at providing a polarization-independent electro-optic device require either complicated technology, or considerable sacrifice in operation voltages and length.
Additional techniques aimed at developing configurations that allow polarization independent functioning of electro-optic switches are disclosed in the following publications:
(6) H. Okayama et al., “Three-Guided Directional Coupler as Polarization Independent Optical Switch”, Electronics Letters, vol. 27, No. 10, pp. 810–812, 1991;
(7) P. J. Duthie et al., “A polarization Independent Guided-Wave LiNbO3 Electrooptic Switch Employing Polarization Diversity”. IEEE Photonics Technology Letters, vol. 3, No. 2, pp. 136–137, 1991;
(8) M. Kondo et al., “Low-Drive-Voltage and Low-Loss Polarization-Independent LiNbO3 Optical Waveguides Switches”, Electronics Letters, vol. 23, No. 21,pp. 1167–1169, 1987; and
(9) P. Granestrand et al., “Polarization Independent Switch and Polarization Splitter Employing Δβ and Δk Modulation”, Electronics Letters, vol. 24, No. 18, pp. 1142–1143, 1988.